PET

Positron emission tomography (PET) scans and combination PET and computed tomography (PET/CT) scans are routinely used to diagnose and treat a variety of cancers.

What is PET?

PET involves the use of an imaging device (a PET scanner) and a radiotracer that is injected into the patient’s bloodstream. A frequently used PET radiotracer is 18F-fluorodeoxyglucose (FDG), a compound derived from a simple sugar and a small amount of radioactive fluorine.

The e FDG radiotracer accumulates in the body’s tissues and organs based on the rate of sugar use. Cancer needs a large amount of energy, which comes from sugar. The PET scanner creates three-dimensional images that show how the FDG is distributed in the areas of the body.

Areas where a large amount of FDG accumulates, called ‘hot spots’ because they appear more intense than surrounding tissue, indicate that a high level of sugar use or metabolism is occurring there. These hot spots are often caused by cancer cells.

PET-CT is a combination of PET and computed tomography (CT) that produces highly detailed views of the body. The combination of two imaging techniques—called co-registration, fusion imaging or hybrid imaging—allows information from two different types of scans to be viewed in a single set of images. CT imaging uses advanced x-ray equipment and in some cases a contrast-enhancing material to produce three dimensional images.

A combined PET-CT study is able to provide detail on both the anatomy and function of organs and tissues. This is accomplished by superimposing the precise location of abnormal metabolic activity from PET against the detailed anatomic image (from CT).

How is PET performed?

The procedure begins with an intravenous (IV) injection of a radiotracer, such as FDG, which usually takes between 30 and 60 minutes to have a fixed distribution throughout the body. The patient is then placed in the PET scanner where special detectors are used to create a three dimensional image of the FDG distribution.

Scans are reviewed and interpreted by a qualified imaging professional such as a nuclear medicine physician or radiologist who shares the results with the patient’s physician.

How is PET used for cancer?

Physicians use PET and PET-CT studies to:

diagnoseand stage: by determining the exact location of a tumor, the extent or stage of the disease and whether the cancer has spread in the body

plan treatment: by selecting the most effective therapy based on the unique molecular properties of the disease and of the patient’s genetic makeup

evaluate the effectiveness of treatment: by determining the patient’s response to specific drugs and ongoing therapy. Based on changes in cellular activity observed on PET-CT images, treatment plans can be quickly altered

manage ongoing care: by detecting the recurrence of cancer

Advantages of PET imaging

PET is a powerful tool for diagnosing and determining the stage of many types of cancer, including lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic and brain cancers. The value of PET for many other cancers is currently being investigated through the National Oncologic PET Registry.

By detecting whether lesions are benign or malignant, PET scans may eliminate the need for surgical biopsy or identify the optimal biopsy location

PET scans help physicians choose the most appropriate treatment plan and assess whether chemotherapy or other treatments are working as intended

PET scans are currently the most effective means of detecting a recurrence of cancer.

Preparing for a PET Scan

You will receive specific instructions on how to prepare based on the type of PET scan you are undergoing. In general, your preparation will involve the following:

You should wear comfortable clothes to your appointment; you may be allowed to wear your own clothing or you may be given a gown to wear during the exam.

If there is any possibility that you are pregnant or you are breastfeeding, you should inform your physician. You may be advised to pump and store breast milk before your procedure to be used after the PET radiopharmaceutical and CT contrast material is no longer in your body.

You should tell your doctor what medications you are taking, if you have any allergies — especially to contrast materials, iodine, or seafood — and any other medical conditions.

Metal objects including jewelry, eyeglasses, dentures and hairpins may affect the CT images and should be left at home or removed prior to your exam. You may also be asked to remove hearing aids and removable dental work.

You will be asked not to eat anything for at least four hours before a whole body PET-CT scan and to retrain from drinking liquids other than water for several hours before the scan. If you are diabetic, you may be given special instructions.

How is the procedure performed?

If necessary, you may have an intravenous (IV) line inserted into a vein in your hand or arm. A dose of radiotracer will be injected intravenously, swallowed or inhaled as a gas.

It will take approximately 60 minutes for the radiotracer to travel through your body and be absorbed. You will be asked to rest quietly during this time. You may also be asked to drink a contrast material that will help the radiologist interpreting the study or to empty your bladder.

When it is time to begin imaging, you will be positioned on an examination table and moved into the PET-CT scanner. You will need to remain still during imaging.

If you are having both a PET and CT exam, the CT scanning is usually done first followed by the PET. The actual CT scanning takes less than two minutes and the PET scan requires between 20 and 30 minutes.

You may be asked to wait until the technologist checks the images in case additional images are needed.